The present invention relates to an apparatus for use in networks which connect a plurality of computers to form a computer network and, more particularly, to apparatus for multiplexing data streams between a master computer and a plurality of slave computers in such a network.
Many businesses have computer networks which extend over large geographic areas. The most economical manner for implementing such networks utilizes existing telephone lines to connect the various computers in the network. Computer networks that utilize the telephone system are known to the prior art. However, these computer networks are less than ideal. Problems arise from the use of transmission facilities which were designed for voice quality transmissions between two points and from the inclusion of components which are owned and maintained by separate commercial entities. However, because of the cost savings which are inherent in the use of the existing telephone system, it is desirable to adapt such network equipment for acceptable digital data transmission, if at all possible.
Existing computer networks suffer from a number of drawbacks. For example, prior art computer networks which utilize the telephone system are difficult to reconfigure either with regard to the speed at which data is transmitted within the telephone system or with regard to the topological configuration of the computer network. In prior art computer networks, telephone company technicians must be dispatched to various locations within the telephone system to affect a rate change in the computer network. If the new rate is not one of a small number of rates for which the equipment is designed, the actual interface circuitry within the telephone system must often be changed. As a result, it is practically impossible to change data rates in response to short term changes in the quality of the telephone lines carrying the data.
Short term increases in the noise on specific telephone lines are common. The telephone lines used for carrying the user's computer data are, in general, identical to the lines used to carry voice communications within the telephone system. These lines are bundled into cables with similar lines. The noise level on these lines depends upon a number of factors including the nature of the transmissions on lines bundled with those used for the computer network along routes which include a variety of different telephone company switching equipment.
To change the network speed or configuration, changes must be made in equipment which is neither owned or maintained by the computer network owner. This makes reconfiguration of the network difficult, since telephone company personnel must be coordinated with those of the network owner to affect a change.
Ideally, a network owner would like to be able to change the configuration of the network in response to observed error rates or the time of the day. Such reconfiguration, for example, would be useful in smoothing peak loads on the computer network. Such reconfigurability would also be useful in transferring specific services to offices in earlier time zones when the office providing the service in another time zone closes at the end of the day. In prior art systems, such reconfiguration requires that key components of the computer network be rewired by telephone company personnel or that special purpose hardware be installed at significant cost.
Another example of a drawback in prior art computer networks is the inability of the user to monitor and manage the computer network. If a failure occurs in the computer network, it is difficult to ascertain whether the failure is the result of a malfunction in the equipment supplied by the telephone company, either on or off of the network owner's premises, or in the equipment owned and maintained by the network owner. Hence, when a malfunction is detected, telephone company technicians are often needed to ascertain the location of the failure within the system. The time delays in waiting for such determinations to be made are often unacceptable. It would be advantageous to provide a means for allowing the network user to set loopbacks and other diagnostic aids within the computer network without the time delays and added expense inherent when telephone company personnel must be involved. Similarly, the computer network user would like to be able to access data showing error rates on the various communication links in the computer network.
A typical multipoint network having a master computer and a plurality of slave computers and utilizing the telephone system requires a form of multiplexing device referred to herein after as a multijunction unit. The multijunction unit combines data streams from various slave computers prior to transmitting the combined data to the master computer. One or more multijunction units are typically located in the telephone system and operate in conjunction with the T.sub.1 carrier facilities of the system.
One problem inherent in combining the data streams from a large number of slave computers is that noise on the individual communication links coupling the slave computers to the master computer is combined as well. At any given time, only one computer is typically transmitting data to the master computer. However, noise on the communication links connecting the slave computers which are not transmitting data to the master computer is still present. This noise is independent of whether or not the slave computer in question is actually transmitting data. In prior art multijunction units, this noise is combined with the data from the active slave computer. As a result, the noise level increases with the number of slave computers in the computer network.
One method of reducing errors on the various communication links within the computer network is to encrypt the data using an error correcting code prior to transmission. However, such error encryption interferes with the "combining" function of a multijunction unit. Hence, it would be advantageous to provide a multijunction unit which could function properly on error encrypted data.
Broadly, it is an object of the present invention to provide an improved computer multijunction unit for use in computer networks utilizing the telephone system.
It is another object of the present invention to allow the use of encrypted data in a manner which achieves an improved overall error rate in the telephone network system without interfering with the combining function of the multijunction unit.
A related object of the present invention is to provide a multijunction unit which prevents noise in the data streams of non-active or non-selected slave channels from being combined with the data stream from the active slave channel.
It is a further object of the present invention to provide a multijunction unit in which the speed of data transmission can be altered automatically in response to observed error rates in the computer network.
These and other objects of the present invention which will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.